Note, though this help page refers to two free downloads, your particular print credit may contain more or less than two images.
Go to the image you wish to download
Log in or Create an account
Redeem Print credit on top menu
Enter your code
You should see something like
If not click view my package
When you can see “Add photo to 2 Free Medium Downloads” then click that to add your photo to the free download package
Select your next photo and repeat the above.
Once you’ve added two free images to the package you should be able to checkout, without any payment required (unless of course you’ve ordered anything else.)